VetteVet - Wiring 2 for single battery diagram

[SilverMiner]

VetteVet - hope you see this, as I have two questions about correctly performing the upgrades shown in the image you frequently share titled "Wiring 2 for single battery" (also attached below).

1st question - for the dark red sensing wire running from the distribution block to the alternator, is it correct to leave the original wiring in place and merely splice into the factory wire coming off the alternator? Or does the factory wire need to be eliminated entirely? What is the correct gauge for the new wire?

2nd question - Is there any harm in using 6 gauge wire from the starter solenoid to the distribution block instead of the 8 guage as shown? If using 6 gauge, should the fusible link then also be changed to 12 gauge?

It appears I might have slipped a couple extra side questions in, but we all greatly appreciate the excellent help you provide to this forum.

Edited to add - I forgot to mention my truck is a 1980 K10 Suburban, 12SI v-belt style 100 amp PowerMaster alternator.